Scientific Highlights 2013

We present a combined magnetic neutron scattering and muon spin rotation study of the nature of the magnetic and superconducting phases in electronically phase separated La_2-xSr_xCuO_4+y, x=0.04, 0.065, 0.09. For all samples, we find long-range modulated magnetic order below T_N ≅ T_c = 39 K. In sharp contrast to oxygen-stoichiometric La_2-xSr_xCuO₄, we find that the magnetic propagation vector as well as the ordered magnetic moment is independent of Sr content and consistent with that of the "striped" cuprates. Our study provides direct proof that superoxygenation in La_2-xSr_xCuO_4+y allows the spin stripe ordered phase to emerge and phase separate from superconducting regions with the hallmarks of optimally doped oxygen-stoichiometric La_2-xSr_xCuO₄.

The structure and properties of the ferromagnet Tb_1-xDy_xFe₂ are explored through the morphotropic phase boundary (MPB) separating ferroic phases of differing symmetry. Our synchrotron data support a first order structural transition, with a broadening MPB width at higher temperatures. The optimal point for magnetomechanical applications is not centered on the MPB but lies on the rhombohedral side, where the high striction of the rhombohedral majority phase combines with the softened anisotropy of the MPB. We compare our findings with single ion crystal field theory and with ferroelectric MPBs, where the controlling energies are different.

The properties of amorphous solids below 1 K are dominated by atomic tunneling systems. A basic description is given by the standard tunneling model. Despite its success, the standard tunneling model still remains phenomenological and little is known about the microscopic nature of tunneling systems in amorphous solids. We present dielectric polarization echo experiments on partially deuterated amorphous glycerol. Nuclear quadrupoles, introduced by the deuteration, influence the echo amplitude in a characteristic way and allow us to draw for the first time detailed conclusions about the microscopic nature of the tunneling processes in amorphous glycerol.

First measurements of the self-dynamics of liquid water in the GPa range are reported. The GPa range has here become accessible through a new setup for the Paris-Edinburgh press specially conceived for quasielastic neutron scattering studies. A direct measurement of both the translational and rotational diffusion coefficients of water along the 400 K isotherm up to 3 GPa, corresponding to the melting point of ice VII, is provided and compared with molecular dynamics simulations. The translational diffusion is observed to strongly decrease with pressure, though its variation slows down for pressures higher than 1 GPa and decouples from that of the shear viscosity. The rotational diffusion turns out to be insensitive to pressure. Through comparison with structural data and molecular dynamics simulations, we show that this is a consequence of the rigidity of the first neighbors shell and of the invariance of the number of hydrogen bonds of a water molecule under high pressure. These results show the inadequacy of the Stokes-Einstein-Debye equations to predict the self-diffusive behavior of water at high temperature and high pressure, and challenge the usual description of hot dense water behaving as a simple liquid.

While propagation of dislocations in body centered cubic metals at low temperature is understood in terms of elementary steps on {110} planes, slip traces correspond often with other crystallographic or non-crystallographic planes. In the past, characterization of slip was limited to post-mortem electron microscopy and slip trace analysis on the sample surface. Here with in-situ Laue diffraction experiments during micro-compression we demonstrate that when two {110} planes containing the same slip direction experience the same resolved shear stress, sharp slip traces are observed on a {112} plane. When however the {110} planes are slightly differently stressed, macroscopic strain is measured on the individual planes and collective cross-slip is used to fulfill mechanical boundary conditions, resulting in a zig-zag or broad slip trace on the sample surface. We anticipate that such dynamics can occur in polycrystalline metals due to local inhomogeneous stress distributions and can cause unusual slip transfer among grains.

G-protein-coupled receptors (GPCRs) are physiologically important membrane proteins that sense signalling molecules such as hormones and neurotransmitters, and are the targets of several prescribed drugs. Recent exciting developments are providing unprecedented insights into the structure and function of several medically important GPCRs. Here, through a systematic analysis of high-resolution GPCR structures, we uncover a conserved network of non-covalent contacts that defines the GPCR fold. Furthermore, our comparative analysis reveals characteristic features of ligand binding and conformational changes during receptor activation. A holistic understanding that integrates molecular and systems biology of GPCRs holds promise for new therapeutics and personalized medicine.

High-temperature superconductivity emerges from an un-conventional metallic state. This has stimulated strong efforts to understand exactly how Fermi liquids breakdown and evolve into an un-conventional metal. A fundamental question is how Fermi liquid quasiparticle excitations break down in momentum space. Here we show, using angle-resolved photoemission spectroscopy, that the Fermi liquid quasiparticle excitations of the overdoped superconducting cuprate La_1.77Sr_0.23CuO₄ is highly anisotropic in momentum space. The quasiparticle scattering and residue behave differently along the Fermi surface and hence the Kadowaki–Woods relation is not obeyed. This kind of Fermi liquid breakdown may apply to a wide range of strongly correlated metal systems where spin fluctuations are present.

Results are presented from a search for the rare decays B_s⁰ → μ⁺μ^- and B⁰ → μ⁺μ^- in pp collisions at √s=7 and 8 TeV, with data samples corresponding to integrated luminosities of 5 and 20 fb^-1, respectively, collected by the CMS experiment at the LHC. An unbind maximum-likelihood fit to the dimuon invariant mass distribution gives a branching fraction B(B⁰ → μ⁺μ^-) = (3.0^+1.0_-0.9)x10^-9, where the uncertainty includes both statistical and systematic contributions. An excess of B_s⁰ → μ⁺μ^- events with respect to background is observed with a significance of 4.3 standard deviations. For the decay B⁰ → μ⁺μ^- < 1.1x10^-9 at the 95% confidence level is determined. Both results are in agreement with the expectations from the standard model.

The analysis of a combined data set, totaling 3.6⋅10¹⁴ stopped muons on target, in the search for the lepton flavor violating decay μ⁺→e⁺γ is presented. The data collected by the MEG experiment at the Paul Scherrer Institut show no excess of events compared to background expectations and yield a new upper limit on the branching ratio of this decay of 5.7⋅10¹³ (90% confidence level). This represents a four times more stringent limit than the previous world best limit set by MEG.

The controlled manipulation of the charge carrier concentration in nanometer thin layers is the basis of current semiconductor technology and of fundamental importance for device applications. Here we show that it is possible to induce a persistent inversion from n- to p-type in a 200-nm-thick surface layer of a germanium wafer by illumination with white and blue light. We induce the inversion with a half-life of ∼12 hours at a temperature of 220 K which disappears above 280 K. The photo-induced inversion is absent for a sample with a 20-nm-thick gold capping layer providing a Schottky barrier at the interface. This indicates that charge accumulation at the surface is essential to explain the observed inversion. The contactless change of carrier concentration is potentially interesting for device applications in opto-electronics where the gate electrode and gate oxide could be replaced by the semiconductor surface.

The S=1/2 spin chain material SrCuO₂ doped with 1% S=1 Ni impurities is studied by inelastic neutron scattering. At low temperatures, the spectrum shows a pseudogap Δ ≈ 8 meV, absent in the parent compound, and not related to any structural phase transition. The pseudogap is shown to be a generic feature of quantum spin chains with dilute defects. A simple model based on this idea quantitatively accounts for the experimental data measured in the temperature range from 2 to 300 K, and allows us to represent the momentum-integrated dynamic structure factor in a universal scaling form.
 

Single phase and strained LuMnO₃ thin films are discovered to display coexisting ferromagnetic and antiferromagnetic orders. A large moment ferromagnetism (≈ 1μB), which is absent in bulk samples, is shown to display a magnetic moment distribution that is peaked at the highly strained substrate-film interface. We further show that the strain-induced ferromagnetism and the antiferromagnetic order are coupled via an exchange field, therefore demonstrating strained rare-earth manganite thin films as promising candidate systems for new multifunctional devices.
 

We report the observation of a stepwise "melting" of the low-temperature Na-vacancy order in the layered transition-metal oxide Na_0.7CoO₂. High-resolution neutron powder diffraction analysis indicates the existence of two first-order structural transitions, one at T₁ ≈ 290 K followed by a second at T₂ ≈ 400 K. Detailed analysis strongly suggests that both transitions are linked to changes in the Na mobility. Our data are consistent with a two-step disappearance of Na-vacancy order through the successive opening of first quasi-1D (T₁ > T > T₂) and then 2D (T > T₂) Na diffusion paths. These results shed new light on previous, seemingly incompatible, experimental interpretations regarding the relationship between Na-vacancy order and Na dynamics in this material. They also represent an important step towards the tuning of physical properties and the design of tailored functional materials through an improved control and understanding of ionic diffusion.
 

We study the structural and magnetic orders in electron-doped BaFe_2-xNi_xAs₂ by high-resolution synchrotron x-ray and neutron scatterings. Upon Ni doping x, the nearly simultaneous tetragonal-to-orthorhombic structural (T_s) and antiferromagnetic (T_N) phase transitions in BaFe_2-xNi_xAs₂ are gradually suppressed and separated, resulting in T_s > T_N with increasing x, as was previously observed. However, the temperature separation between T_s and T_N decreases with increasing x for x ≥ 0.065, tending toward a quantum bicritical point near optimal superconductivity at x ≈ 0.1. The zero-temperature transition is preempted by the formation of a secondary incommensurate magnetic phase in the region 0.088 ≤ x ≤ 0.104, resulting in a finite value of T_N ≈ T_c + 10K above the superconducting dome around x ≈ 0.1. Our results imply an avoided quantum critical point, which is expected to strongly influence the properties of both the normal and superconducting states.
 

Damascening, defined in this context as the inlay of one metal into a different metal base, is a rare decorative technique in the Early Bronze Age, known only from seven bronze artefacts found north of the Alps. This paper reports on the first thorough scientific examination of one such find, the axe from Thun-Renzenbühl grave no. 1. This interdisciplinary project involving several institutions in Germany and Switzerland investigated the axe by means of neutron radiographic imaging and X-ray microprobe methods, supported by microscopic examination. The result is an attempt to reconstruct the fabrication and decoration process and to reconsider the enigmatic question of the origins of the damascene technique north of the Alps.
 

Despite the great interest organic spintronics has recently attracted, there is only a partial understanding of the fundamental physics behind electron spin relaxation in organic semiconductors. Mechanisms based on hyperfine interaction have been demonstrated, but the role of the spin-orbit interaction remains elusive. Here, we report muon spin spectroscopy and time-resolved photoluminescence measurements on two series of molecular semiconductors in which the strength of the spin-orbit interaction has been systematically modified with a targeted chemical substitution of different atoms at a particular molecular site. We find that the spin-orbit interaction is a significant source of electron spin relaxation in these materials.

Birefringence measurements at high magnetic field strength of up to 33 T were used to detect magnetically induced alignment of bicelles composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), cholesterol, and 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine-diethylenetriaminepentaacetate (DMPE-DTPA) with complexed lanthanide ions. These birefringence measurements together with a small-angle neutron scattering (SANS) analysis in a magnetic field showed parallel alignment of the bicelles if the lanthanide was thulium (Tm³⁺), and perpendicular alignment with dysprosium (Dy³⁺). With the birefringence measurements, the order parameter S can be determined as a function of the magnetic field strength, if the magnetic alignment reaches saturation. Additional structural information can be obtained if the maximum induced birefringence is considered. The degree of alignment of the studied bicelles increased with decreasing temperature from 40 to 5 °C and showed a new bicellar structure comprising a transient hole formation at intermediate temperatures (20 °C) during heating from 5 to 40 °C.

The importance of surfactant self-assemblies in foam stabilization is well-known. The aim of the current study was to investigate the self-assemblies of the nonionic surfactant polyglycerol ester (PGE) in bulk solutions, at the interface and within foams, using a combined approach of small-angle neutron scattering, neutron reflectivity, and electron microscopy. PGE bulk solutions contain vesicles as well as open lamellar structures. Upon heating of the solutions the lamellar spacing increases, with significant differences in the presence of NaCl or CaCl₂ as compared to the standard solution. The adsorption of the multilamellar structures present in the bulk solutions lead to a multilayered film at the air–water interface. The ordering within this film was increased as a result of a 20% area compression mimicking a coalescence event. Finally, PGE foams were shown to be stabilized not only by strong interfacial films but also by agglomerated self-assemblies within the interstitial areas of the foams.

We have used low-energy implanted muons as a volume sensitive probe of the magnetic properties of EuO_1-x thin films. We find that static and homogeneous magnetic order persists up to the elevated TC in the doped samples, and the muon signal displays the double dome feature also observed in the sample magnetization. Our results appear incompatible with either the magnetic phase separation or bound magnetic polaron descriptions previously suggested to explain the elevated T_C, but are compatible with an RKKY-like interaction mediating magnetic interactions above 69 K.

The vanadium oxyfluoride [NH₄]₂[C₇H₁₄N][V₇O₆F₁₈] (DQVOF) is a geometrically frustrated magnetic bilayer material. The structure consists of S=1/2 kagome planes of V⁴⁺ d¹ ions with S=1 V³⁺ d² ions located between the kagome layers. Muon spin relaxation measurements demonstrate the absence of spin freezing down to 40 mK despite an energy scale of 60 K for antiferromagnetic exchange interactions. From magnetization and heat capacity measurements we conclude that the S=1 spins of the interplane V³⁺ ions are weakly coupled to the kagome layers, such that DQVOF can be viewed as an experimental model for S=1/2 kagome physics, and that it displays a gapless spin liquid ground state.

Based on high-field ³¹P nuclear magnetic resonance experiments and accompanying numerical calculations, it is argued that in the frustrated S=1/2 ladder compound BiCu₂PO₆ a field-induced soliton lattice develops above a critical field of μ₀H_c1=20.96(7)T. Solitons result from the fraction- alization of the S=1, bosonlike triplet excitations, which in other quantum antiferromagnets are commonly known to experience Bose-Einstein condensation or to crystallize in a superstructure. Unlike in spin-Peierls systems, these field-induced quantum domain walls do not arise from a state with broken translational symmetry and are triggered exclusively by magnetic frustration. Our model predicts yet another second-order phase transition at H_c2>H_c1, driven by soliton-soliton interactions, most likely corresponding to the one observed in recent magnetocaloric and other bulk measurements.

We report the low-temperature electronic and magnetic properties of the alkali metal-organic solvent intercalated iron selenide superconductor Li(C₅H₅N)_0.2Fe₂Se₂ using muon-spin-spectroscopy measurements. The zero-field muon spin relaxation (μSR) results indicate that nearly half of the sample is magnetically ordered and spatially phase separated from the superconducting region. The transverse-field μSR results reveal that the superfluid density of Li(C₅H₅N)_0.2Fe₂Se₂ is two dimensional in nature. The temperature dependence of the penetration depth λ(T) can be explained using a two-gap s-wave model. This implies that, despite the 2D nature of the superfluid density, the symmetry of the superconducting gap remains unaltered to the parent compound FeSe.

Schmieröl ist für Motoren lebenswichtig. Auch Mehrscheibenkupplungen, wie sie in Motorrädern zu finden sind, werden mit Öl geschmiert und gekühlt. Gleichzeitig muss die Ölpumpe aber vom Motor angetrieben werden und erhöht damit den Kraftstoffverbrauch des Fahrzeugs. Das Ziel der Entwickler ist deshalb, die Antriebskomponenten optimal zu schmieren – mit einem möglichst kleinen Ölvolumenstrom. Um zu sehen, wie sich das Öl innerhalb einer Kupplung verteilt, arbeiten die Entwickler der Schaeffler-Marke LuK (D) mit den Wissenschaftlern des Paul Scherrer Instituts in Villigen (CH) zusammen, die mit Neutronen Metalle durchleuchten.

The magnetic structure and electronic ground state of the layered perovskite Ba₂IrO₄ have been investigated using x-ray resonant magnetic scattering. Our results are compared with those for Sr₂IrO₄, for which we provide supplementary data on its magnetic structure. We find that the dominant, long-range antiferromagnetic order is remarkably similar in the two compounds and that the electronic ground state in Ba₂IrO₄, deduced from an investigation of the x-ray resonant magnetic scattering L₃/L₂ intensity ratio, is consistent with a J_eff=1/2 description. The robustness of these two key electronic properties to the considerable structural differences between the Ba and Sr analogues is discussed in terms of the enhanced role of the spin-orbit interaction in 5d transition metal oxides.

From magnetic, specific heat, ¹⁷⁰Yb Mössbauer effect, neutron diffraction, and muon spin relaxation measurements on polycrystalline Yb₂Sn₂O₇, we show that below the first order transition at 0.15 K all of the Yb³⁺ ions are long-range magnetically ordered and each has a moment of 1.1μ_B which lies at ≈ 10° to a common fourfold cubic axis. The four sublattice moments have four different directions away from this axis and are therefore noncoplanar. We term this arrangement splayed ferromagnetism. This ground state has a dynamical component with a fluctuation rate in the megahertz range. The net ferromagnetic exchange interaction has an anisotropy that favors the local threefold axis. We discuss our results in terms of the phase diagram proposed by Savary and Balents [Phys. Rev. Lett. 108, 037202 (2012)] for a pyrochlore lattice of Kramers 1/2 effective spins.

The low-field phase of the organic superconductor (TMTSF)₂ClO₄ is studied by muon-spin rotation. The zero temperature limit of the magnetic penetration depth within the TMTSF layers is obtained to be λ_ab = 0.86(2)μm. Temperature dependence of the muon-spin relaxation shows no indication of gap nodes on the Fermi surface nor of any spontaneous fields due to time-reversal-symmetry breaking. The weight of evidence suggests that the symmetry of this low-field phase is odd-frequency p-wave singlet, a novel example of odd-frequency pairing in a bulk superconductor.

Muon-spin rotation measurements, performed on the mixed state of the classic anisotropic superconductor Bi_2.15Sr_1.85CaCu₂O₈, obtain quantities directly related to two- and three-body correlations of vortices in space. A novel phase diagram emerges from such local probe measurements of the bulk, revealing an unusual glassy state at intermediate fields which appears to freeze continuously from the equilibrium vortex liquid but differs both from the lattice and the conventional high-field vortex glass state in its structure.

Lithium migration in nanostructured electrode materials is important for an understanding and improvement of high energy density lithium batteries. An approach to measure lithium transport through nanometer thin layers of relevant electrochemical materials is presented using amorphous silicon as a model system. A multilayer consisting of a repetition of five [⁶LiNbO₃ (15 nm)/Si (10 nm)/^natLiNbO₃ (15 nm)/Si (10 nm)] units is used for analysis, where LiNbO₃ is a Li tracer reservoir. It is shown that the change of the relative ⁶Li/⁷Li isotope fraction in the LiNbO3 layers by lithium diffusion through the nanosized silicon layers can be monitored nondestructively by neutron reflectometry. The results can be used to calculate transport parameters.

Magnetic clusters, i.e., assemblies of a finite number (between two or three and several hundred) of interacting spin centers which are magnetically decoupled from their environment, can be found in many materials ranging from inorganic compounds and magnetic molecules to artificial metal structures formed on surfaces and metalloproteins. Their magnetic excitation spectra are determined by the nature of the spin centers and of the magnetic interactions, and the particular arrangement of the mutual interaction paths between the spin centers. Small clusters of up to four magnetic ions are ideal model systems in which to examine the fundamental magnetic interactions, which are usually dominated by Heisenberg exchange, but often complemented by anisotropic and/or higher-order interactions. In large magnetic clusters, which may potentially deal with a dozen or more spin centers, there is the possibility of novel many-body quantum states and quantum phenomena. In this review the necessary theoretical concepts and experimental techniques to study the magnetic cluster excitations and the resulting characteristic magnetic properties are introduced, followed by examples of small clusters, demonstrating the enormous amount of detailed physical information that can be retrieved. The current understanding of the excitations and their physical interpretation in the molecular nanomagnets which represent large magnetic clusters is then presented, with a section devoted to the subclass of single-molecule magnets, distinguished by displaying quantum tunneling of the magnetization. Finally, there is a summary of some quantum many-body states which evolve in magnetic insulators characterized by built-in or field-induced magnetic clusters. The review concludes by addressing future perspectives in the field of magnetic cluster excitations.

Diluted magnetic semiconductors have received much attention due to their potential applications for spintronics devices. A prototypical system (Ga,Mn)As has been widely studied since the 1990s. The simultaneous spin and charge doping via hetero-valent (Ga³⁺,Mn²⁺) substitution, however, resulted in severely limited solubility without availability of bulk specimens. Here we report the synthesis of a new diluted magnetic semi- conductor (Ba_1-xK_x)(Zn_1-yMn_y)₂As₂, which is isostructural to the 122 iron-based superconductors with the tetragonal ThCr₂Si₂ (122) structure. Holes are doped via (Ba²⁺,K¹⁺) replacements, while spins via isovalent (Zn²⁺,Mn²⁺) substitutions. Bulk samples with x = 0.1-0.3 and y = 0.05-0.15 exhibit ferromagnetic order with T_C up to 180 K, which is comparable to the highest T_C for (Ga,Mn)As and significantly enhanced from T_C up to 50 K of the ‘111’-based Li(Zn,Mn)As. Moreover, ferromagnetic (Ba,K)(Zn,Mn)₂As₂ shares the same 122 crystal structure with semiconducting BaZn₂As₂, antiferromagnetic BaMn₂As₂ and superconducting (Ba,K)Fe₂As₂, which makes them promising for the development of multilayer functional devices.

Accurate knowledge of the charge and Zemach radii of the proton is essential, not only for understanding its structure but also as input for tests of bound-state quantum electrodynamics and its predictions for the energy levels of hydrogen. These radii may be extracted from the laser spectroscopy of muonic hydrogen (μp, that is, a proton orbited by a muon). We measured the 2S_1/2^F=0 - 2P_3/2^F=1 transition frequency in μp to be 54611.16(1.05) gigahertz (numbers in parentheses indicate one standard deviation of uncertainty) and reevaluated the 2S_1/2^F=1 - 2P_3/2^F=2 transition frequency, yielding 49881.35(65) gigahertz. From the measurements, we determined the Zemach radius, r_Z = 1.082(37) femtometers, and the magnetic radius, r_M = 0.87(6) femtometer, of the proton. We also extracted the charge radius, r_E = 0.84087(39) femtometer, with an order of magnitude more precision than the 2010-CODATA value and at 7σ variance with respect to it, thus reinforcing the proton radius puzzle.

The MuCap experiment at the Paul Scherrer Institute has measured the rate Λ_S of muon capture from the singlet state of the muonic hydrogen atom to a precision of 1%. A muon beam was stopped in a time projection chamber filled with 10-bar, ultrapure hydrogen gas. Cylindrical wire chambers and a segmented scintillator barrel detected electrons from muon decay. Λ_S is determined from the difference between the μ^- disappearance rate in hydrogen and the free muon decay rate. The result is based on the analysis of 1.2 ⋅ 10¹⁰ μ^- decays, from which we extract the capture rate Λ_S=(714.9 ± 5.4_stat ± 5.1_syst)s^-1 and derive the proton’s pseudoscalar coupling g_P(q₀² = -0.88m_μ²) = 8.06 ± 0.55.

In the tetragonal heavy fermion system CeCoIn₅ the unconventional superconducting state is probed by means of muon spin rotation. The pressure dependence (0-1 GPa) of the basal-plane magnetic penetration depth (λ_a), the penetration depth anisotropy ( γ = λ_c/λ_a ) and the temperature dependence of 1/λ_i² (i=a,c) were studied in single crystals. A strong decrease of λ_a with pressure was observed, while γ and λ_i²(0)/λ_i²(T) are pressure independent. A linear relationship between 1/λ_a²(270mK) and T_c was also found. The large decrease of λ_a with pressure is the signature of an increase of the number of superconducting quasiparticles by a factor of about 2.